The Medipix1 chip is a prototype (digital) CMOS imaging chip that emerged from particle detection in high energy physics experiments. It was designed at CERN following specifications from the Medipix1 Collaboration. This contribution aims at demonstrating the applicability of Medipix1 Si and GaAs pixel devices for alpha and X-ray particle detection. A study of “charge sharing” between pixels and test of “spatial resolution” using narrow beam and “edge” image contrast was performed. For photons with energy <35 keV charge sharing between neighbouring pixels is negligible. On the contrary alpha-particles are detected in the form of clusters of adjacent pixels with typical cluster diameter of about 2.5 pixels. Possible applications of the device for position sensitive heavy charged particle detection (e.g. for surface analysis or for neutron detection) are considered.